In the prior art of information display, coded information has been decoded or interpreted selectively for enabling use of the code bit patterns to have different functions in operation of the display. For example, display systems of the prior art have included a source of coded data, plural utilization devices for the data, and means to steer the data to one or another of the utilization devices. One utilization device may be a character generator, another an "attribute" decoder. An "attribute" may be some control for the display, such as a field marker to cause following characters to be treated in a given way, such as to be displayed with higher than usual brightness, etc. Whether the bytes of coded data were to represent characters or attributes has been determined by examination of their bit patterns. A flag bit or bit combination would identify a byte as an attribute, and therefore that bit combination would not be available to represent a display character.
Display systems usually employ data bytes of a given size, and this limits the number of distinct codes or "code points" available. For example, an eight bit byte is ordinarily limited to representation of one of a total of 256 characters and attributes. If a character set is to include a large number of symbols, representing numbers, punctuation, upper and lower case alphabetic characters, specially accented characters or compound characters for various languages, and so on, the number of characters in the set can easily approach 256. If at the same time, the number of kinds of attributes or other control bytes desired is substantial, the available number of distinct codes may be exceeded.
This problem is complicated by the fact that in many systems, one or two bits act as "flags" to distinguish between displayable characters and attributes. If a high order "1" bit signals an attribute, than half the byte set is lost for displayable characters; even if the two highest order bits must be a "1", a quarter of the set is lost.